An Air Force B-52 bomber crashed off Guam, killing at least two airmen and leading to the search of a vast area of the Pacific Ocean for the remaining four crew members, the US military said.

Six vessels, three helicopters, two F-15 fighter jets and a B-52 bomber were involved in the search, which had covered about 70 square miles of ocean, said Coast Guard spokeswoman Lt Elizabeth Buendia.

The Navy, Coast Guard, Air Force and local fire and police departments were involved with the search.Major Stuart Upton, a Pentagon spokesman, said the aircraft was unarmed.

The B-52 bomber, based at Barksdale Air Force Base in Louisiana, was en route to conduct a flyover in a parade when it crashed about 30 miles northwest of Apra Harbour, the Air Force said.

The Liberation Day parade celebrates the day when the US military arrived on Guam to retake control of the island from Japan.The Air Force said a board of officers will investigate the accident.

The accident is the second for the Air Force this year on Guam, a US territory 3,700 miles southwest of Hawaii.In February, a B-2 crashed at Andersen Air Force Base shortly after takeoff in the first-ever crash of a stealth bomber. Both pilots ejected safely. The military estimated the cost of the loss of the aircraft at 1.4 billion dollars.

The B-52 is a long-range, heavy bomber that can refuel in mid air. Since the 159-foot-long bomber was first placed into service in 1955, it has been used for a wide range of missions from attacks to ocean surveillance. Two B-52s, in two hours, can monitor 140,000 square miles of ocean surface.